This invention relates to radios, tunable bandpass filters used in radios, and specifically to automatically tuning a bandpass filter.
Tunable bandpass filters are used in RF (radio frequency) and IF (intermediate frequency) stages of radio equipment such as transmitters and receivers. FIG. 1 is an exemplary block diagram of a radio receiver 10 employing a RF tunable bandpass filter 15 at the input to the receiver 10 connected between an antenna 12 and a low-noise amplifier 17. A IF tunable filter 20 is also used connected between a mixer 18 and an IF amplifier 22 or elsewhere within a receiver IF. A local oscillator or frequency synthesizer 16 provides the injection for mixer 18 for down conversion to IF. The tunable bandpass filters 15 and 20 provide needed selectivity to reduce image responses, interference, and spurious signal response problems brought about by strong adjacent frequency signals. The IF output is processed by a digital signal processing and control processor 25 where additional filtering, decimation, and demodulation are performed. A transmitter (not shown) with tunable bandpass filters may also be implemented with RF tunable bandpass filters and IF tunable bandpass filters. Bandpass filters in transmitters are used to reduce radiation of undesired signals such as harmonics, spurious responses, etc.
Tunable bandpass filters require time and effort by a test technician to adjust during production test for use in a product. Tunable bandpass filters may age over the lifetime of a radio degrading performance and may require periodic realignment by a technician. Manual tuning and adjustment of tunable bandpass filters by a technician can be costly.
Typically tunable bandpass filters are tested and aligned by injecting a test signal and monitoring the magnitude of the test signal at an output of the tunable filter. Monitoring the magnitude is straight forward and easily implemented. Unfortunately, peak magnitude of tunable bandpass filters can vary over the filter tunable bandwidth. If the passband of the tunable bandpass filter is wide enough, magnitude information is not enough to find the center of the passband unless the test signal is a swept frequency source.
What is needed is a method and apparatus that reduces or eliminates extensive manual tuning of tunable bandpass filters in radio equipment to reduce the cost of the radio while improving performance by utilizing processing power in the radio.